You search for a §, followed with anything but a blank and store it to $1. Then you replace it by itself surrounded by brackets. The modifier g means that you do it as often as possible.
Update: Thanks to i5513 and MidLifeXis for their improvements of my regex and for polishing my rusty knowledge! I like that in PerlMonks: by answering questions you often learn something for yourself, too :-)

Thank you so much, but here comes the tricky part:
I have to match "§" and in the replacement to use "& # 0167;"
otherwise - either does not match for "& # 0167;",
or replacing with "§" in it - brings strange prints